ceph/src/boost/libs/metaparse/doc/except.qbk

   1 [#except]
   2 [section except]
   3
   4 [h1 Synopsis]
   5
   6   template <class P, class Result, class ErrorMsg>
   7   struct except;
   8
   9 This is a [link parser_combinator parser combinator].
  10
  11 [table Arguments
  12   [[Name]       [Type]]
  13   [[`P`]        [[link parser parser]]]
  14   [[`Result`]   [[link metaprogramming_value template metaprogramming value]]]
  15   [[`ErrorMsg`] [[link parsing_error_message parsing error message]]]
  16 ]
  17
  18 [h1 Description]
  19
  20 `except` accepts the input when `P` rejects it and the result of parsing is the
  21 `Result` argument. When `P` accepts the input, `except` rejects it and the
  22 reason is `ErrorMsg`.
  23
  24 [h1 Header]
  25
  26   #include <boost/metaparse/except.hpp>
  27
  28 [h1 Expression semantics]
  29
  30 For any `p` parser, `c` class, `msg` parsing error message, `s` compile-time
  31 string and `pos` source position the following are equivalent
  32
  33   get_result<except<p, c, msg>, s, pos>::type
  34   c
  35
  36   get_remaining<except<p, c, msg>, s, pos>::type
  37   s
  38
  39   get_position<except<p, c, msg>, s, pos>::type
  40   pos
  41
  42 when `p` rejects the input. The result of the parser is an error with the error
  43 message `msg` otherwise.
  44
  45 [h1 Example]
  46
  47   #include <boost/metaparse/except.hpp>
  48   #include <boost/metaparse/int_.hpp>
  49   #include <boost/metaparse/string.hpp>
  50   #include <boost/metaparse/start.hpp>
  51   #include <boost/metaparse/get_result.hpp>
  52   #include <boost/metaparse/get_message.hpp>
  53   #include <boost/metaparse/define_error.hpp>
  54
  55   #include <type_traits>
  56
  57   using namespace boost::metaparse;
  58
  59   BOOST_METAPARSE_DEFINE_ERROR(
  60     number_is_not_allowed,
  61     "numbers are not allowed here"
  62   );
  63
  64   using except_int =
  65     except<int_, std::integral_constant<int, 1>, number_is_not_allowed>;
  66
  67   static_assert(
  68     get_result<
  69       except_int::apply<BOOST_METAPARSE_STRING("foo"), start>
  70     >::type::value == 1,
  71     "it should accept the input when it is not an integer"
  72   );
  73
  74   static_assert(
  75     std::is_same<
  76       number_is_not_allowed,
  77       get_message<except_int::apply<BOOST_METAPARSE_STRING("13"), start>>::type
  78     >::type::value,
  79     "it should reject the input when it is an integer"
  80   );
  81
  82 [endsect]
  83